The present invention generally relates to broadband networks. More specifically, the invention is related to an inverting line driver architecture having reduced distortion and improved power efficiency.
With the advancement of technology and the need for instantaneous information, the ability to transfer digital information from one location to another, such as from a central office (CO) to a customer premises (CP), has become more and more important. Allowing for increasing data transmission rates has become a requirement, as opposed to an option.
As part of the system responsible for proper transmission and reception of data in a broadband network, an analog front end (AFE) interfaces between an analog transmission medium, such as an analog telephone line, and digital signal processing circuits. The AFE, among other functions, converts a digital signal into a continuous time analog signal. Particular to the case of a digital subscribe line (DSL) AFE with an integrated line driver, the AFE also puts this converted signal on a two-wire pair. The AFE circuit performs this function by using the combination of a digital-to-analog converter and an analog-to-digital converter. The digital-to-analog converter receives digital signals from a digital signal processor (DSP), and converts them to analog signals, upon which the analog signals are transmitted to a line driver. The analog-to-digital converter receives analog signals, converts them to digital signals, and furnishes them to the DSP. The AFE may also incorporate other elaborate analog signal processing.
Once the analog signal is transmitted to the line driver, the line driver drives the analog signal through the two-wire pair in accordance with a required power particular to the application considered and the type of line driven. Most established DSL applications have a required standard power spectral density template that they are required to meet. As an example, for the most common DSL service currently deployed (2B1Q HDSL), the required transmit power is approximately 13.5 dBm. This yields a maximum peak power of 18.6 dBm, for a peak current of 48.2 mA (assuming 6 Vpp differential output voltage, 3 dB loss in back matching resistors, and a peak-to-average ratio (PAR) of 1.8 for 2B1Q).
At the other extreme, the required transmit power for a Discrete Multi Tone-Asymmetric Digital Subscriber Line-Central Office (DMT-ADSL-CO), assuming 3 dB loss in the back matching resistors and a PAR of 5.3 for DMT, is approximately 20.5 dBm. This yields a maximum peak power of 37.98 dBm, for a peak current of 2095 mA (assuming a 6 Vpp differential output voltage). It should be noted that there is a multitude of other applications at various power levels between these two extremes. Also, due to line impairment caused by a variety of different factors, particularly bridge taps, the actual line impedance might be significantly less then expected, and the load current can thus be significantly higher then expected.
For DSL systems, the line driver is typically the most significant source of distortion in the data transmit path, due to the high speed and large load current requirement, combined with the variety of line configurations encountered. For most high-speed ADSL applications the trend is currently to avoid transmitting and receiving in the same frequency bands. From the customer premise side, typically there is transmission at the low frequency end of the band and reception at the upper frequency end of the band. Thus, distortion products, which fall at multiples of the transmit-signal, fall in the receive spectrum. Typically, distortion in the transmit-path couples into the receive path through a hybrid, which makes the receiver implementation significantly more challenging. The receiver needs to recover the portion of the transmit-signal, which falls in the receive-band, as well as the receive-signal by itself. Therefore, limitation of distortion in the transmit-path is important to both the transmission and reception of information.
It should also be noted, that distortion introduced in the receive-band by any element in the transmit path, up to the line driver, can be filtered out with a passive external LC filter before it is fed to the line driver. However, if the line driver introduces the distortion, it is not practical to filter the distortion out due to the low impedance level. Thus, the distortion introduced by the line driver is the final limiting factor for the distortion of the transmit-path, which typically is the most critical factor in terms of achievable reach of the system.
Typical line driver implementation is based on high input impedance amplifiers. This makes it easy to interface to the line driver. However, it implies that there is significant signal swing across the input terminals of the line driver. This changes the common mode input to the line driver, which inherently changes the gain, in turn, yielding distortion and hampering transmitted and received data.
Since the conventional line driver architecture is non-inverting, it is impossible to introduce voltage attenuation in this stage of signal transmission. Thus, if voltage attenuation is desired, it must be performed at a previous stage in the signal chain. This will, however, reduce the signal to noise ratio of the overall signal path, since the noise will not be reduced when the signal is reduced. Most of the noise is introduced before the line driver, and the less voltage gain needed at this stage, the better the overall signal to noise ratio achieved. Further, for some ADSL applications, standards allow for power cutback for short loop operation. For DMT ADSL, the appropriate standards are ITU G.992.1 for G-Lite, and ITU G.992.2 and ANSI T1.413 for G-Heavy. Thus, the system would need to be able to attenuate the signal significantly below the standard magnitude. If this can be done at the final stage of the line driver, it can be achieved with no additional digital signal processing, and no cost in terms of signal to noise ratio. Further, if the attenuation had to be performed in the digital domain, by reducing the analog input signal going into the AFE, the signal to noise (SNR) ratio would drop proportional to the reduction of the signal level.
It should also be noted, that an amplifier, such as the high input amplifiers of the line driver, will have a higher closed loop bandwidth if it has a low gain. Thus, for optimum overall performance, maximum closed loop bandwidth, and as little closed loop gain as possible, is desired.
Also, current line drivers are typically barely capable of delivering the required power into an ideal load. As an example, if the line driver were designed for DMT ADSL customer premises applications operating at 7 Vpp output swing, the required peak current would be 300 mA in order to generate the required 13.5 dBm into a 100-Ohm line. In order to achieve a power efficient design, the design would be done assuming a maximum output current of about 350 mA. However, in the case of bridge taps, when the effective load is cut in half and the maximum required current therefore could be about 600 mA, significant distortion results. In order to address this problem, an increase in the drive capability of the line driver could be implemented. This would, however, reduce the speed of the device, given the same quiescent power, and increase the distortion during normal load conditions. Thus, the same amplifier cannot be optimized readily to handle both scenarios.
Therefore, the system designer typically has to weigh trade-offs between maximum line power, distortion, and quiescent current in order to find the line driver most suitable for a given application. If power consumption were most significant, the designer may pick a line driver that is barely capable of delivering the required power, but can do this efficiently with little distortion. However, if the device is connected to a line with a bridge tap, such that it is asked to deliver twice the nominal line current, significant distortion will be observed, and the line will come down. Alternatively, the designer could design the system to be able to handle bridge taps on all lines, but would then not be able to meet the power budget.
Bipolar drivers are also used to circumvent this problem. These cannot, however, be integrated readily into a single chip AFE, as it is impractical to perform the other functions of the AFE in pure bipolar technology. The bipolar devices also typically have significantly higher power consumption than their CMOS counterparts due to the higher supply voltage and inability of bipolar devices to go close to either supply. Most CMOS line drivers typically can go within less then 1V from either power supply. Bipolar drivers, however, typically rely on Darlington output stages, which makes it impossible to get closer than about 2V from either supply rail.
Therefore, there is a need in the industry for an amplifier architecture which is specifically suited for large drive, high speed applications, and which can achieve an optimized output stage for any load condition without causing distortion or utilizing excessive amounts of power.
Briefly described, the invention is a line driver implemented within an analog front end, which utilizes an inverting amplifier architecture with programmable open loop gain, programmable closed loop gain and attenuation, programmable maximum output drive, and which eliminates distortion otherwise introduced by gain variation caused by common mode input variations.
In general, a first embodiment of the invention provides for a line driver having an input stage and an output stage. The input stage of the line driver is identified by the deriving of the open loop gain of a preamplifier. The output stage is defined by the deriving of the open loop gain of two drivers that provide the power required for the line driver to drive a two-wire pair. Both, the preamplifier of the input stage, and the drivers of the output stage, have inputs that sit at a common mode voltage, thereby inhibiting a common mode input voltage swing and limiting distortion in the line driver.
The closed loop gain of the line driver may be changed to a desired value by regulating the values of resistors therein, thereby providing for the obtaining of a desired gain, regardless of whether the gain value is below or above 1. The size of output devices in the drivers are programmable under digital control to achieve an optimum size for a given drive requirement for a given digital subscriber line application and line. Digital programmability of the output drive allows the line driver to be reconfigured by digital control, so as to provide for the driving of the two-wire pair, regardless of unexpected impedances, such as a bridge tap.
The invention has numerous advantages, a few of which are delineated hereafter as examples. Note that the embodiments of the invention, which are described herein, possess one or more, but not necessarily all, of the advantages, set out hereafter.
One advantage of the invention is that the input common-mode of the preamplifiers, as well as the output drivers, of the line driver is not a function of the input signal, but instead, is fixed at the independently controlled and applied common mode voltage, VCM.
Another advantage of the inverting amplifier architecture is that it enables the closed loop gain of the line driver to be less then one.
Another advantage of the invention is that it provides programmable open loop gain.
Another advantage of the line driver is that it provides for programmable drive capability, wherein the system adjusts the maximum possible output current of the output driver according to the application and two-wire pair conditions, to obtain the optimum maximum current.